J . Phys. Chem. 1993,97, 11175-1 1179
11175
Effect of the Ligand-Field Strength on the Radiative Properties of the Ligand-Localized 37rr?r* State of Rhodium Complexes with 1,lO-Phenanthroline. Proposed Role of dd States Hisayuki Miki, Masashi Shimada, and Tohru Azumi' Department of Chemistry, Faculty of Science, Tohoku University. Sendai 980, Japan
J. A. Brozik and G. A. Crosby' Department of Chemistry, Washington State University, Pullman, Washington 99164-4630 Received: May 7 , 1993; In Final Form: August 20, 1993'
In order to quantify the effect of metal d-orbitals on the radiative properties of the triplet spin sublevels of the ligand-localized 3aa*state, we have studied the phosphorescence from the ligand-localized 3 a a * state of cisRh(CN)z(phen)zCl and Rh(phen)3Cl3 in the crystalline state at 1.3 K by optical detection of magnetic resonance (ODMR). The ODMR experiments show that only the T,-spin sublevel is emissive for cis-Rh(CN)z(phen)zCl, whereas the T, and T,sublevels are both emissive for Rh(phen)3Cl3. These experimental results are satisfactorily interpreted in terms of the modulation of the configurational mixing of singlet and triplet d a * and dd states with the phen-localized aa* and UT* states induced by changes in ligand-field strength.
Introduction Luminescence from metal complexes with the (nd)6 configuration has received substantial attention from both spectroscopists and photochemists. The interest stems from the capacity to generate species in which the lowest (emitting) state can be dictated by a judicious choice of metal atom, oxidation state, and coordinated ligands. Complexes containing 2,2'-bipyridine (bpy), 1,lO-phenanthroline (phen), 2-phenylpyridine (phpy), 242thieny1)pyridine (thpy), and benzo[h]quinoline(bhq) have been extensively studied.'-' The nature of the luminescence from such complexes is generally classified into three types. The observed phosphorescence can arise from a dd state, RhClz(phen)l; from a da* state, Ru(phen)32+; or from a UT* state, Rh(~hen)3'+.~ The first two types possess sublevels that are too far separated to be amenable to investigations by optical detection of magnetic resonance (ODMR). The3aa* ligand-localizedlevelin a complex retains many of its free ligand characteristics, however, and much can be learned from the application of microwave techniques on polycrystalline samples at sub-4 K temperatures where spinlattice relaxation is inhibited. The emission from Rh(~hen)3~+ is certainly aa* in nature. This assignment was first made on the basis of the obvious similarities of both the location and the structure of the phosphorescence band observed from a glass at 77 K to those of the free ligand,? although the lifetime is significantly shorter (>102) than that of the free ligand.9 Thus, it is clear that coordination to the metal introduces new radiative pathways to the ground state. The 3aa* nature of the emitting state was confirmed by Komada et aL,1 who carried out detailed investigations of the sublevels of Rh(phen)P+ and also of the analogous Rh(bpy)i+ complex by the ODMR method. Recently, the 3 ~ ~ state of Rh(phpy)z(bpy)+ was investigated by Frei et a1.6bby polarized absorption spectroscopy. In both of these investigations the effect of metal coordinationon the radiative properties of the 3ua* state was attributed to spin-orbit coupling between the %a* and a Ida* state in the complex. Their analyses invoked da* states but did not include any contribution from dd states. Indirect evidence for the mixing of dd states with the emitting %a* term leading to modifications of its sublevel characteristics has been obtained in our laboratories.10 A study (77 K glasses) of five pairs of rhodium complexes with the generic formulas *Abstract published in Aduance ACS Abstracts, October 1, 1993.
Rh(s-phen)33+and Rh(CN)Z(s-phen)z+,wheres-phenis a methylsubstituted phenanthroline, revealed that in all cases cyanide substitution increased the lifetime of the observed phosphorescence. We believe that these results are a manifestation of the substantial displacementof interacting dd states to higher energy due to the cyanide ions, as expected from the position of cyanide vis-a-visphenanthrolinein the spectrochemical series. Moreover, direct evidence for the existenceof a dd level lying slightly above the emitting3as* statein Rh(~hen)~'+ has beenpublished." The most convincing evidence, however, for the incidence of a dd level near the 3aa* emitting state in these types of molecules has appeared recently; a grow-in of add phosphorescence with increase in temperature has been documented.12 In addition, these authors show that cyanide substitution raises the dd levels so high that they are not thermally accessible from the 3aa* level even at room temperature. There is electrochemicaland chemical evidence that a vacant d-orbital on the rhodium ion lies near the LUMO of the 1,lOphenanthrolineligand. Negative cyclovoltammetry scans for the rhodium species are irreversible.10 We interpret these results to indicate that the added electron partially occupies the d-orbital and causes dissociation of the complex. It is significant that the scans for the analogous ruthenium species are reversible, however. For the latter molecules it is known that the d-orbital of the ion lies significantly above the LUMO of the coordinated ligand. Finally, we note that the standard methods of synthesizing rhodium tris complexes employ reducing agents and that Ru(I1) is a known oxidation state. Thus, chemical evidence also points toward the location of a vacant d-orbital on the rhodium ion near the LUMO of the phen ligand. In the current study we compare the radiative propertiesof the * spin sublevels of the 3a7r*emitting state of phen, Rh(phen)s3+, and Rh(CN)*(phen)z+. Our intent is to elucidate the role of metal d-orbitals in modifying the sublevel characteristics. We approach the problem by exploiting the shift in ligand-field states that occurs upon replacement of one bidentate phen ligand with two cyanides and relating the consequent changes in sublevel characteristics to the relative importance of one-center spinorbit-coupling integrals on the rhodium atom.
Experimental Section cis-Rh(CN)n(phen)zC1was synthesized as reported" and was recrystallized from water; Rh(phen)3Cl3 was synthesized in an
0022-3654193 12097-11175%04.00/0 0 1993 American Chemical Society
Miki et al.
11176 The Journal of Physical Chemistry, Vol. 97, No. 43, 1993
(b) 2.0- 2.2 GHz
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x 10’~ Figure 1. Phosphorescence spectrum of ci~-Rh(CN)z(phen)2Clin the polycrystalline state at 1.3 K. WAVENUMBER
oxygen-free atmosphere from 1,10-phenanthrolinethat had been purified by vacuum sublimation.10 Spectra from polycrystalline samples were recorded. Phosphorescence spectra were observed with a Spex 1102 monochromator equipped with a Hamamatsu R3896 photomultiplier tube. The 313-nm line from a 500-W high-pressure Hg lamp was used for excitation. For the decay measurements excitation was carried out by means of a N2-pulsed laser, and the resultant signal was digitized by a Kawasaki Electronica TMR10 transient memory. The zero-field optical detection of magnetic resonance (zfODMR) was performed by monitoring the 0-0 band of the phosphorescence. The equipment used for this investigation was essentially the same as that reported previ0us1y.l~ Zero-field splittingsof the excited triplet state were determined by phosphorescence microwave double resonance (PMDR) measurements.’s Total decay-rate constants and relative radiative-rate constants for the individual sublevels were determined by the fast-passage technique.16 Total decay-rate constants for the nonemissive sublevels were determined by the method of microwave induced delayed phosphorescence (MIDP).”
Results The phosphorescence spectra of Rh(CN)z(phen)zCl and Rh(phen)sC13 are essentially identical, in their locations and structures, to the previously reported spectrum of Rh(phenh(BF4)3. 1 Since the phosphorescence of polycrystallineRh (CN)2(phen)~Clhas not yet been published, we include the spectrum observed at 1.3 K in Figure 1. In an ethanol glass at 77 K the lifetimes were exponential. Values of 58 ms for cis-Rh(CN)z(phen)zCl and 30 ms for Rh(phen)$lp were obtained. In the polycrystalline state at 4.2 and 1.3 K,however, the phosphorescence decay curves were not single exponentials for either compound, indicating that spin-lattice relaxation is at least partially suppressed at these temperatures. For Rh(phen)$13 we were able to obtain all three ODMR signals. Our measured zero-field splittings, total decay-rate constants, and relative radiative-rate constants essentially agree with those reported previous1y.l For cis-Rh(CN)z(phen)zCl, on the other hand, we were able to detect only two ODMR signals, one at 2.09 and a second at 4.58 GHz. The fast-passage signals, shown in Figure 2, are both single exponentials; each has the same decay-rate constant of 40 s-1, The single exponentiality of the fast-passage signals indicates either (a) that the radiativerate constants of the two sublevels are drastically different or (b) that the total decay-rate constants of the two sublevels are both 40 s-I. MIDP measurements indicated that the decay-rate constants of the less emissive sublevels were 11 s-1 for both the observed transitions. These results rule out case b. Due to the rather noisy fast-passage signals, the ratios of the radiative-rate constants for each pair of sublevels were not determined accurately. We can only say that the ratio is less than 0.2 for both cases. The occurrenceof two sublevelswith essentially equal
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Figure2. Fast-passagesignalsofcis-Rh(CN)~(phen)2CI. The resonance frequencies are (a) 4.58 and (b) 2.09 GHz.
decay rates is consistent with our inability to observe a third ODMR signal. The total decay-rate constants of the three sublevels of Rh(CN)2(phen)z+ that are compatible with these experiments are either (a) 40,40, and 11 s-1 or (b) 40, 11, and 11 s-1. We chose the latter set, because the average of 40, 11, and 11 s-1 is close to the single decay-rate constant observed at 77 K (17.2 s-l).I8 The properties of the excited triplet sublevels thus determined are shown in Figure 3. We note that the T, sublevel is the most radiative in phen14 and the T, sublevel is the most radiative in ci~-Rh(CN)2(phen)~Cl. The Tz and T, sublevels in Rh(phen)sC13 have comparable radiative activities. On an absolute scale the complexes have far greater radiative activities than the free ligand.
Discussion The decay rates show that the radiative properties of the phenlocalized 3 7 r ~ *level are significantly enhanced by coordination to Rh3+. This observation clearly implies a mixing of metal orbitals into the essentially phen-localized states. A theoretical analysisof the participation of metal orbitals in the phen-localized states can be carried out in two ways. One approach is the molecular-orbital method in which the excited-state properties are analyzed in terms of the entire molecule. Currently, however, there does not appear to exist any molecular-orbital method that can satisfactorily interpret the excited-state properties of complexes containing an atom as heavy as rhodium. Therefore, we adopt a composite molecule theory,lg in which the excited states of a complex are expressed as a mixing of the ligand-localized (m* and U T * ) configuration with metal ligand-field (dd) and metal-to-ligand charge-transfer (d**) configurations. In the treatment that follows we assume that the 3 ~ excitation ~ * is completelylocalizedon one of the phen ligands, as was confirmed for R h ( b ~ y ) 3 ~by+ Westra and Glasbeek.2O We further assume that the entire complex can be treated within the CZ,point group even though the ground-state symmetry is either 4 [for Rh(phen)s3+] or CZ [for ci~-Rh(CN)2(phen)z+]. Thus the phenlocalized, the charge-transfer, and the ligand-field states are all treated in the C, point group. The z-principal axis lies along the short axis of the excited phen ligand, and the x-axis is chosen perpendicular to the phen plane (see Figure 4). Figure 4 includes the symmetries of the phen-localized ‘K and ‘K* orbitals and the occupied (tz,) and unoccupied (e,) d-orbitals of rhodium (in Oh). The occupied orbitals belong to al, a2, and bl, and the unoccupied orbitals belong to a1 and b2 in C., The spatial part of the lowest 3 m * state of phen belongs to the B2 representation and mainly consists of the configuration arising from electron promotion from the bl HOMO to the a2* LUMO. Phenanthroline-Ligand Properties. We begin by examining the individual sublevels of the 3 B z ( ~ ~state * ) of free phen. Their radiative properties were discussed in a previous paper.14 The
The Journal of Physical Chemistry, Vol. 97, No. 43, 1993 11177
Ligand-Localized 3 r r * State of Rhodium Complexes k d d g(0-0,re])
T(77 K)
kdd
kds-' g(0-0,rel)
g(0-0rel) ,
11
